Replace heavy std::string/iostream usage in the DLMS parser with stack-allocated char buffers and snprintf to avoid heap allocations and reduce runtime overhead. Introduce buffer-based helpers (data_to_string_, obis_to_string_) and switch hex formatting to format_hex_pretty_to; remove <sstream>/<iomanip> and include <cstdio>. Update header signatures accordingly. Also change DlmsPushComponent to use App.get_loop_component_start_time() instead of millis() when tracking last_rx_char_time_ for RX timeout handling.
Files changed: components/dlms_push/dlms_parser.cpp/.h (buffer-based string helpers, logging, include changes), components/dlms_push/dlms_push.cpp (use App loop time for timeout).
Note: The string helper APIs were converted to buffer-oriented versions — update any callers to provide an output buffer.
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